I just made a bag layed out like this for my UI..
To get around the sizing problem I hid slots 2,4,6,8,10,12,14,16,18 in UI Builder and set there OnShow event to resize the window to what I needed..
EX Code (From my UI) for slot 2 OnShow property
Code:
Parent.MinimumSize=96,112
Parent.MaximumSize=96,112
Parent.Size=96,112
Remember to set the Visible property for those slots to false..